The present invention relates to a gallium phosphide light emitting device and a method for fabricating thereof, and in more detail, a gallium phosphide light emitting device with a high luminance and a method for fabricating such device.
Light emitting devices such as light emitting diodes are generally obtained by stacking a plurality of semiconductor layers on a semiconductor substrate to thereby fabricate a multi-layered semiconductor wafer having a p-n junction, and processing such wafer into the devices. Among such devices, those having a dominant emission wavelength (dominant wavelength) within a range from 555 to 580 nm are obtained by stacking on an n-type gallium phosphide (which may simply be noted as xe2x80x9cGaPxe2x80x9d hereinafter) substrate at least one each of n-type and p-type gallium phosphide layers in this order, and processing the obtained wafer into the devices. A method for determining the dominant emission wavelength (dominant wavelength) is defined by JIS(Japanese Industrial Standard)-Z8701 (1995).
While GaP having no doped nitrogen (N) serving as an emission center can emit a green light with a dominant emission wavelength of 559 nm or longer and shorter than 560 nm, the light emission efficiency is considerably low since the emission is ascribable to indirect transition. Adding nitrogen (N) to such GaP can now significantly increase the light emission efficiency. It has been considered that such increase is mainly contributed by a mechanism mentioned below. That is, N capable of substituting phosphorus (P) in GaP, will become an electrically neutral impurity since N is a group V element same as P. N has, however, a larger electron affinity than P has, so that it can trap a neighboring electron. Such impurity is known as an isoelectronic trap. Once an electron is trapped, Coulomb""s force appears and is exerted far beyond to attract a hole, which results in generation of an exciton. Dissipation by recombination of such exciton emits a green to yellow-green light with a dominant emission wavelength between 560 nm and 580 nm, where energy of such light nearly corresponds to the band gap. Light having a wavelength in the above range and in particular between 560 nm and 564 nm is substantially recognized as a green light. It is now generally known that a potential of N relative to an electron is expressed as a narrow and deep short-distance force in the real space, so that a wave function of the electron can extend widely in the momentum space. Such extension of the wave function in the momentum space desirably increases direct transition component with a null wave vector, so as to allow GaP to have a relatively large emission-recombination probability despite of its nature of indirect transition.
FIGS. 9A and 9B respectively show an exemplary sectional constitution and a carrier concentration profile of the individual layers of a conventional GaP light emitting device. As shown in FIG. 9A, the conventional GaP light emitting device has on an n-type GaP single-crystalline substrate 40 an n-type GaP crystallinity improving layer 41, a non-N-doped n-type GaP layer 42, an N-doped n-type GaP layer 43, an N-doped p-type GaP layer 44, and a non-N-doped p-type GaP layer 45 stacked in this order. A p-n junction is formed between the N-doped n-type GaP layer 43 and the N-doped p-type GaP layer 44. The n-type GaP substrate 40 and the n-type GaP layers 41 to 43 are added with silicon (Si) as an n-type dopant, and the p-type GaP layers 44 and 45 with zinc (Zn) as a p-type dopant.
Such constitution of the GaP light emitting device shown in FIG. 9A, having the N-doped p-n junction for composing a light emissive zone allows a higher light emission efficiency and accordingly a higher luminance as compared with a device having no doped N. It should now be noted that luminance refers to brightness per unit area of a light emitting body. Since luminance considerably varies depending on the morphology of the light emitting body or measurement methods, the value will be evaluated on a relative basis in the context of the present invention.
The light emission efficiency of the GaP light emitting device having the N-doped p-n junction portion is, however, still smaller than that, for example, of a GaP: Znxe2x80x94O red LED, so that it is important to grow a p-n junction made of high-quality crystal having a minimum amount of defects in order to raise the luminance. The N-doped p-type GaP layer 44 is most commonly grown by the so-called impurity compensation process. The process is such that growing the n-type GaP layer 43, adding zinc (Zn) or other acceptors into the gallium solution, and then growing the p-type GaP layer 44 while compensating the donor. The process is advantageous in that allowing formation of the p-n junction portion using a single solution.
It is now noted that liquid-phase growth of the N-doped n-type GaP layer 43 composing the p-n junction requires ammonia gas as an source to be introduced into the growth vessel containing the Si-added gallium solution. On the other hand, growth of the N-doped p-type GaP layer 44 is proceeded by further adding Zn to the above Si-added gallium solution to thereby compensate the donor impurity (Si), and by similarly introducing ammonia gas. Adding N both to the n-type GaP layer 43 and the p-type GaP layer 44 can principally ensure both layers with a probability of light emission. The p-type GaP layer 44 grown by the foregoing impurity compensation process will, however, inevitably has the acceptor concentration higher than the donor concentration in the n-type GaP layer 43, so that the emission from the n-type GaP layer 43 upon hole injection will become dominant.
In the process of the N doping using ammonia gas, Si reacts with ammonia to produce a stable compound to thereby lower the Si content in the gallium solution, which results in lowered Si concentration (donor concentration, accordingly carrier concentration) in the N-doped n-type GaP layer 43 as shown in FIG. 9B. Since a donor that presents in the light emissive zone acts as a non-emissive center, it is important to prevent as much as possible the donor concentration from being lowered, in order to extend a lifetime of the carrier. The foregoing introduction of ammonia gas during the growth of the n-type GaP layer 43 in order to reduce the concentration of Si as a donor is thus advantageous in terms of correspondingly raising the injection efficiency of holes responsible for light emission.
As described in the above, adding N to the GaP layer composing the p-n junction successfully raises the light emission efficiency. Raising the N concentration, however, cause red-shift of the dominant emission wavelength to intensify yellow component, so that it has been difficult to further raise luminance at a stable wavelength.
In addition, while the ammonia gas introduction during the growth of the n-type GaP layer 43 inevitably reduces the Si (donor) concentration, to thereby advantageously raise the injection efficiency of holes responsible for the light emission, this will raise another problem in that the donor concentration of the n-type GaP layer 43 will always vary in association with the concentration of N introduced in a form of ammonia gas and serves as a light emissive center, so that it will become difficult to control the carrier concentration independently from the N concentration.
It is therefore an object of the present invention to solve the foregoing problems and to provide a gallium phosphide light emitting device with a high luminance and a method for fabricating such device.
A major concept of the present invention resides in that providing a nitrogen-doped low carrier concentration layer having both of a donor concentration and an acceptor concentration controlled below 1xc3x971016/cm3 at a p-n junction portion between an n-type GaP layer and a p-type GaP layer, to thereby raise the luminance by as much as 20 to 30% over the conventional luminance. Such technique has never been tried by the prior art such as growing the p-type GaP layer composing the p-n junction portion by the impurity compensation process, and the present inventors are the first to clarify effects of such technique.
Suppressing the donor concentration and the acceptor concentration in the low carrier concentration layer below 1xc3x971016/cm3 inevitably gives a carrier concentration, which is expressed as a difference between both concentrations, lower than 1xc3x971016/cm3 accordingly. It should now be noted that matters of importance are suppressing the concentration of the donor which serves as a non-emissive center below 1xc3x971016/cm3 to thereby extend the carrier lifetime; and that concomitantly suppressing the carrier concentration at a level significantly lower than that of the adjacent layer, more specifically below 1xc3x97106/cm3, to thereby raise the emission efficiency upon injection of electrons or holes.
The low carrier concentration layer contains sulfur and carbon as dopants, where controlling the carbon concentration higher than the sulfur concentration ensures the low carrier concentration layer to have a conduction type of xe2x80x9cpxe2x80x9d, by which the light emission upon electron injection becomes dominant. Since mobility of electrons is far larger than that of holes, this ensures a large electron injection efficiency and an improved luminance.
The reason why sulfur is used as an n-type dopant in the low carrier concentration layer is that sulfur has a relatively high vaporizing pressure and thus can readily be removed by vaporization if the inner pressure of the growth vessel is reduced. The sulfur content in the gallium solution can be reduced if the amount of volatilized sulfur is increased by enhancing the evacuation or by continuing the process under reduced pressure for a longer duration. Carbon used as a p-type dopant of the low carrier concentration is supplied through elution from a carbon-made growth vessel. As the temperature of the gallium solution becomes lower, the amount of carbon eluted into the gallium solution becomes fewer. That is, the carbon concentration and sulfur concentration can be controlled independently from N concentration by properly selecting process conditions under reduced pressure or temperature of the gallium solution.
The luminance of the gallium phosphide light emitting device of the present invention varies depending on the thickness of the low carrier concentration layer, which becomes maximum when the thickness is within a range from 3 to 20 xcexcm.
Providing a non-N-doped low carrier concentration layer between an n-type GaP layer and a p-type GaP layer per se is disclosed in Japanese Laid-Open Patent Publication 6-342935. The publication is, however, aimed at fabricating a GaP light emitting device capable of emitting a pure green light (peak wavelength: approx. 555 nm), hence there is no description suggesting a technique for combining such low carrier concentration layer with N doping. Or rather, N doping into the low carrier concentration layer will red-shift the dominant emission wavelength, which is disadvantageous in terms of obtaining a pure green light emission. Thus the prior art disclosed in the above publication essentially lacks a motivation of combining the low carrier concentration layer and N doping.
On the other hand, N doping to an n-type GaP layer is described in National Technical Report Vol. 25, No. 6, p.1152-1158 (1979) and Japanese Laid-Open Patent Publication 54-53974, where both publications deal with a case in which a large amount of n-type dopant remains in the layer to raise the carrier concentration above 1xc3x971016/cm3 and to achieve a conduction type of xe2x80x9cnxe2x80x9d, which cannot improve the emission efficiency to a satisfiable level. At least the publications do not disclose a GaP layer obtained by doping N into a low carrier concentration layer mainly composed of p-type GaP.
Japanese Laid-Open Patent Publication 57-93589 discloses a GaP light emitting diode in which a p-n junction is formed between an n-type GaP layer and a low carrier concentration p-type GaP layer having a carrier concentration lower than that of such n-type GaP layer (that is, a p-type Gap layer added with an impurity as a shallow donor to thereby have a lowered net acceptor concentration). Again such GaP light emitting diode is unsuccessful in obtaining a green light of high luminance, since the low carrier concentration p-type GaP layer (Zn-doped) attains the low carrier density thereof by being added with a shallow donor such as sulfur (S) in a concentration range from approx. 1xc3x971016/cm3 to 3xc3x971017/cm3.